Metal chelate extraction systems

The carboxylate extraction system has been recognized as more advantageous than the chelate extraction system, since one can deal with more concentrated metal solutions in the former than in the latter. Thus the application of the carboxylate extraction to hydrometallurgy has been attempted, and in this connection extensive studies have been carried out in the Soviet Union and the United Kingdom. 

As stated earlier the polymeric species are often involved in the extraction of metal carboxylates. Therefore, the extraction equilibrium is sometimes more complicated than in the chelate extraction system. As is evident from the following treatment, it is advantageous and often indispensable to study the total metal concentration in the organic phase [Eq. (8)] instead of the conventionally utilized distribution ratio of the metal [Eq. (7)]. 

To date little or no thermodynamic modeling of the phase behavior of the ligand/C02 or metal chelate/C02 systems has been conducted. However, in order for supercritical fluid extraction to be considered as a possible replacement for organic solvent extraction, accurate models must be developed to predict the phase behavior of these systems to allow for both equipment and process design. Equation of state (EOS) modeling was chosen here to model the vapor-liquid equilibrium of the P-diketone/C02 systems studied. Cubic EOSs are the most widely used in modeling high pressure and supercritical fluid systems. This is... 

In the case of inorganic solutes we are concerned largely with samples in aqueous solution so that it is necessary to produce substances, such as neutral metal chelates and ion-association complexes, which are capable of extraction into organic solvents. For organic solutes, however, the extraction system may sometimes involve two immiscible organic solvents rather than the aqueous-organic type of extraction. 

The best way to take advantage of the organic solvent effect without simultaneously diluting the sample is by employing solvent extraction. By this method the element to be analyzed can actually be concentrated and a solution of the element is obtained in essentially pure organic solvent. One of the most commonly used systems involves formation of the metal chelate with ammonium 1-pyrro-lidinecarbodithioate (APDC) and then extracting this into methylisobutyl ketone (MIBK). APDC chelates of many elements form and extract into MIBK from acid solution. 

Efficiency of Extraction. Selectivity of Extraction. Extraction Systems. Extraction of Uncharged Metal Chelates. Methods of Extraction. Applications of Solvent Extraction. 

The use of oxonium and other non-chelated systems can be advantageous where relatively high concentrations of metals are to be extracted as solubility in the organic phase is not likely to be a limiting factor. Metal chelates, on the other hand, have a more limited solubility and are more suited to trace-level work. 

Therefore, the distribution ratio of B remains constant only if the ratio of the activity coefficients is independent of the total concentration of B in the system, which holds approximately in dilute solutions. Thus, although solutions of metal chelates in water or nonpolar organic solvents may be quite nonideal, Nernst s law may still be practically obeyed for them if their concentrations are very low (JCchehte< 10" ). Deviations from Nernst s law (constant D ) will in general take place in moderately concentrated solutions, which are of particular importance for industrial solvent extraction (see Chapter 12). 

Another of the new techniques for extractive preconcentration, separation, and/or purification of metal chelates, biomaterials, and organic compounds is based on the use of surfactant micellar systems. 

Method. Hie metal chelates are prepared by extracting the metal ion from aqueous solution with 20-, 20- and 10-ml volumes of chloroform after addition of an appropriate amount of a solution of DDTC [0.22S g of the sodium salt in 75 ml of water and 25 ml of an ammonia-ammonium nitrate (1 1) buffer, 3 M in total ammonia]. The exact volumes which are used depend on whether the metal is uni-, bi- or tri-valent. The combined chloroform extracts are diluted to at least 50 ml for chromatography. The metal chelates are separated on plates of silica gel G or N which have been activated at 110 °C for 1 h. The Rp values of a number of DDTC metal chelates in a variety of solvent systems are listed in Table 4.31. The dried plates are sprayed with a solution consisting of 1 10 4A/ Pd(II), 7.0 10 5Af calcein and 0.02Mphosphate buffer [dihydrogen phosphate-hydrogen phosphate (1 1)]. This solution must be allowed to stand for 12 h in order to ensure that equilibrium is attained. For quantitative work with low concentrations, the solution of DDTC should be washed with chloroform before use. This removes fluorescent impurities which may cause interference in the chromatography. 

Vp and VL are the volumes of the extraction agent and the liquid sample, respectively, and Kle - Ce/Ce is the distribution coefficient. In practice, an extraction yield higher than 99% is usually considered to be quantitative. With the use of the same volumes of the extraction agent and the sample, this result can be obtained even in a single extraction step if Kle < 0.01. Sometimes the entire procedure can be complicated by a chemical reaction taking place, e.g., in the extractive alkylation (see p.59) or in the preparation of volatile metal chelates (see p.194), and the total yield of the extraction then involves, in addition to the interphase distribution of the initial compounds and products, also the chemical equilibrium which is attained by the reaction. If the quantitative yield of the extraction cannot be predicted on the basis of the character of the system, the extraction efficiency must be determined, otherwise the quantitative evaluation is questionable. 

Based upon the use of nonionic surfactant systems and their cloud point phase separation behavior, several simple, practical, and efficient extraction methods have been proposed for the separation, concentration, and/or purification of a variety of substances including metal ions, proteins, and organic substances (429-441. 443.444). The use of nonionic micelles in this regard was first described and pioneered by Watanabe and co-workers who applied the approach to the separation and enrichment of metal ions (as metal chelates) (429-435). That is, metal ions in solution were converted to sparingly water soluble metal chelates which were then solubilized by addition of nonionic surfactant micelles subsequent to separation by the cloud point technique. Table XVII summarizes data available in the literature demonstrating the potential of the method for the separation of metal ions. As can be seen, factors of up to forty have been reported for the concentration effect of the separated metals. 

The phase separation of nonionic micellar solutions above the cloud point has been succesfully applied to the liquid-liquid extraction of some metal chelate complexes (5, 6J. In these systems the concentration of the analyte takes place in the micellar rich layer, which can be readily analyzed. 

There have been a number of developments in the APDC—MIBK extraction system. In one of them Kinrade and Van Loon [9] used two chelating agents, APDC and DDDC. They maintained that the DDDC has a stabilising effect on all the metal complexes in the system. In this method, they adjusted the pH of 200 ml of aqueous sample with 4 ml of citrate buffer (1.2M sodium citrate and 0.7 M citric acid) to around pH 5.0. 5 ml of the chelating solution (1% (m/v) each of APDC and DDDC in water) and 35 ml (or less) of MIBK are added and the extraction is carried out as described earlier. By this... 

Efficiency of extraction. Selectivity of extraction. Extraction systems. Extraction of uncharged metal chelates. Methods of extraction. Applications of solvent extraction. 

Carbon-based sorbents are relatively new materials for the analysis of noble metal samples of different origin [78-84]. The separation and enrichment of palladium from water, fly ash, and road dust samples on oxidized carbon nanotubes (preconcentration factor of 165) [83] palladium from road dust samples on dithiocarbamate-coated fullerene Cso (sorption efficiency of 99.2 %) [78], and rhodium on multiwalled carbon nanotubes modified with polyacrylonitrile (preconcentration factor of 120) [80] are examples of the application of various carbon-based sorbents for extraction of noble metals from environmental samples. Sorption of Au(III) and Pd(ll) on hybrid material of multiwalled carbon nanotubes grafted with polypropylene amine dendrimers prior to their determination in food and environmental samples has recently been described [84]. Recent application of ion-imprinted polymers using various chelate complexes for SPE of noble metals such as Pt [85] and Pd [86] from environmental samples can be mentioned. Hydrophobic noble metal complexes undergo separation by extraction under cloud point extraction systems, for example, extraction of Pt, Pd, and Au with N, A-dihexyl-A -benzylthiourea-Triton X-114 from sea water and dust samples [87]. 

In (23-40) the polymeric species in both phases have been neglected. As mentioned above, such species may be of special importance in certain systems for metal halide extractions, but are of less importance in chelate extractions. 

The currently applied extraction systems are mainly based on classical extractants, as for example cation exchanging acids or chelating agents, solvating ketones, ethers or esters for metal ion extraction and amines or quaternary ammonium salts for anion extraction [16, 18, 19]. 

The use of chelate complexes with PAR and derivative spectrophotometry allows selective determination of Ni in mixture with Co and Cu [2]. Flow-injection spectrophotometry and PAR were used to determine Ni in silicates and alloys [3]. The detection limits of 77 ng ml was achieved. An online solid-phase extraction system with polyurethane foam was used for separation of Ni from interfering metals (Fe, Cu, Zn and Co). Online analysis of Ni and Cu in industrial effluents using the complexes with PAR has been described [4]. 

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